Group IIB-VIA compound semiconductors in general and cadmium telluride (CdTe) in specific are well-established photovoltaic materials. Several different techniques have been used in the past to prepare CdTe polycrystalline thin films, and photovoltaic devices have been demonstrated using such thin layers.
For example, U.S. Pat. No. 4,388,483 granted to B. M. Basol et al. and assigned to Monosolar Inc., describes the fabrication of a CdS/CdTe solar cell where the thin CdTe film is obtained by a cathodic compound electroplating technique. In this patent, a method is taught where as-deposited n-type CdTe films are type-converted to form rectifying junctions with the CdS coated substrates.
U.S. Pat. No. 4,207,119 granted to Yuan Sheng Tyan and assigned to Kodak Co. describes a CdS/CdTe solar cell produced by a close-spaced sublimation method.
U.S. Pat. No. 4,338,362 by R. Turcotte and assigned to Radiation Monitoring Devices teaches the growth of CdTe films by the spray pyrolysis technique.
Other common methods such as evaporation, sputtering, and chemical vapor deposition have also been employed in preparing thin polycrystalline films of CdTe.
In all of the above-mentioned techniques, the CdTe compound is formed on a substrate which is heated during the process of film deposition. The substrate temperature is typically over 300.degree. C., except in the electrodeposition method where the electrolyte temperature is maintained at about 90.degree. C.
An alternative approach to compound film formation is to first deposit a composite layer of elemental components of the desired compound on a substrate, and then to react these elemental components to form the compound. For example, a screen printing method as applied to CdTe formation (H. Uda et al. in the Proceedings of the 16th IEEE Photovoltaic Specialists' Conference, 1982, pp. 801-804) uses a paste made of mainly Cd and Te powders. This paste is first screen printed on the substrate in the form of a thick film, and then it is dried and sintered at temperatures of about 600.degree. C. to promote a reaction between the Cd and Te powders.
Another example of a process where previously deposited elemental components of CdTe are reacted to form a thin film of this compound is reported in a recent paper by M. J. Carter et al. published in the Proceedings of the 19th IEEE Photovoltaic Specialists' Conference (1987, pp. 1275-1278). In this work, researchers have evaporated four alternate layers of Te and Cd on a substrate which was cooled down to about 10.degree. C. They then reacted these elemental layers by exposing them to pulsed radiation from an argon laser. The elemental layers were each about 700 to 1,000 .ANG. thick. Cooling the substrate was necessary to avoid any premature interaction between the Cd and Te layers before the laser processing. It was found that any such interaction would inhibit the transformation of the whole film into CdTe during the laser processing step. This method could be a good research tool in a laboratory, but it clearly is not a practical, low-cost approach for solar cell production.
Although the attention of the photovoltaics community has so far been concentrated on CdTe as the most important Group IIB-VIA compound semiconductor for solar cell applications, other compounds from the same family of materials offer new possibilities for the production of even higher efficiency cells. For example, cadmium zinc telluride (Cd.sub.x Zn.sub.1-x Te) and mercury zinc telluride (Hg.sub.x Zn.sub.1-x Te) ternaries can be prepared with varying stoichiometries to obtain optical bandgap values between 1.6 and 1.8 eV, and these ternaries can be utilized as top cell materials in high efficiency tandem solar cell structures in which the bottom cell may have a copper indium diselenide (CuInSe.sub.2), a mercury cadmium telluride (Hg.sub.x Cd.sub.1-x Te) or a Hg.sub.x Zn.sub.1-x Te absorber with a bandgap value between 0.9 and 1.2 eV. It should be noted that the bandgaps of the ternary tellurides can be easily tuned to the desired values by changing their stoichiometries.
Most of the reported work on Hg.sub.x Cd.sub.1-x Te and Hg.sub.x Zn.sub.1-x Te thin films has been carried out for infrared detector applications. This work involves growing epitaxial layers of these materials using techniques such as MOCVD (Metallorganic Chemical Vapor Deposition), LPE (Liquid Phase Epitaxy), and MBE (Molecular Beam Epitaxy). Crystalline Cd.sub.x Zn.sub.1-x Te films are also useful as substrate materials for infrared detectors. However, there has been very limited work on the growth of polycrystalline layers of the ternary tellurides for solar cell applications. In U.S. Pat. No. 4,629,820, B. M. Basol et al. describe a compound electrodeposition method which yields Cd-rich Hg.sub.x Cd.sub.1-x Te films and solar cells. Cd.sub.x Zn.sub.1-x Te films of differing stoichiometries have been grown by the evaporation method disclosed by Kimmerle et al. (Thin Solid Films, Vol. 126, pp. 23-29, 1985) for solar cell applications Chu et al. have used the direct combination of gaseous elements to obtain ZnTe and Cd.sub.x Zn.sub.1-x Te polycrystalline layers (J. Appl. Phys., Vol. 59, pp. 1259-1263, 1986).
Doping control is very important for any semiconductor processing technique. The electrical and sometimes optical properties of semiconductors are strong functions of their doping levels Group IIB-VIA compound semiconductors such as Cd.sub.x Zn.sub.1-x Te can be doped p-type or n-type by introducing various dopants into these materials. Possible p-type dopants for Group IIB-VIA compounds are Cu, Ag, Au, N, P, As, Sb, Bi, 0, and the alkali metals. Excess Te would also act as an acceptor in the Group IIB-IVA tellurides. Common n-type dopants include B, Al, Ga, In, Tl, and halogens. Excess Cd, Zn, or Hg also act as donors.
Doping control is generally difficult in thin film processes. In the evaporation method, for example, dopants have to be generally co-evaporated along with the compound. In such a process, the dopant concentration in the deposited film is a complex function of the evaporation rates of the dopant and the compound, and it is also a function of the substrate temperature and even the system geometry. Repeatability in such a doping approach is very poor. In some methods, such as close-spaced sublimation, already doped source materials may be used to obtain doped thin films. The dopants, in this approach, are transferred from the source material into the growing compound film.
In addition to the ability to yield good quality material, there are two other major factors determining the feasibility of a given thin film solar cell fabrication process. These factors are the scalability of the process and its cost. The capital equipment cost, rate of film deposition, processing temperatures, and the utilization of materials are some of the important factors contributing to the cost of a thin film solar cell. The rate of deposition for the close-spaced sublimation technique, for example, is very high but this method is difficult to scale up. It also requires a pre-synthesized compound as the source material, which increases the cost. Compound electrodeposition is a simple technique but its deposition rate is quite low, being typically around 1 to 2 .mu.m per hour. Spray pyrolysis is also a slow technique. Screen printing is attractive, in that it starts with the powders of the low-cost elemental components of the compound which can be deposited on the substrate through a relatively simple method. The drawback of this technology, however, is that it needs relatively thick layers of compounds to avoid pinholes and voids in the deposited films.
From this review of prior art, it is apparent that there is a need for an alternative technique which is simple, versatile and which has the capability of producing thin films of binary and ternary tellurides of Group IIB elements, doped or undoped, in an economical way. Consequently, an object of the present invention is to provide an inexpensive method of producing thin layers of the tellurides of Group IIB elements with various compositions. Another object of the present invention is to provide a simple technique to dope the compound layers. Still another object of the present invention is to provide a method of processing photovoltaic devices using these compounds.